1. Field of the Invention
This invention is directed to disinfection systems and methods, and, more particularly, to such disinfection systems and methods that produce and utilize a mist comprising ozone and other reaction products.
2. Related Art
Ozone is an unstable molecule consisting of three oxygen atoms. An ozone molecule will naturally decay to a single oxygen atom and an oxygen molecule containing two oxygen atoms. If the single oxygen atom does not come into contact with another single oxygen atom and merge with it to form an oxygen molecule, the single oxygen atom will oxidize any inorganic molecule that it comes into contact with and any organic molecule that is susceptible to oxidation that it comes into contact with.
Ozone's half-life in air due to thermal decay at room temperature is about three days; in clean water, the half-life is about 30 min. In practical applications, however, the half-life is much less because of wall effects, humidity, organic loading, and catalytic reactions. Because it is unstable, with a short half-life, ozone must be generated where it is to be used. Ozone may be generated by exposing dry oxygen or a dry gas containing oxygen, such as air, to ultraviolet light, or a high voltage electric field that is corona discharging at the surface of the conductors.
Exposing a surface that is contaminated with one or more biological contaminants, such as bacteria, virus, fungus, fungus spores, yeast, and/or other microorganisms, to ozone will disinfect the surface. Likewise, exposing a confined gas, such as air, that contains biological contaminants to ozone will disinfect the gas. This is because ozone, upon coming into contact with a biological contaminant, initially oxidizes the sheath of the biological contaminant, thereby inactivating it. If the oxidization process continues to completion, the ozone will typically convert the biological contaminant to essentially harmless byproducts, such as water and carbon dioxide. This is an advantageous method for sterilizing a surface or a gas because the process is simple and the end products of the oxidization, water and carbon dioxide, and the end product of the ozone decay, molecular oxygen, are harmless.
The principal difficulties with using ozone to disinfect a surface or a gas are: (1) only weak concentrations of ozone may be achieved in a practical application of ozone to a surface or a volume of gas because of ozone's short half-life in practical applications, and (2) ozone is injurious to humans at the concentrations required to disinfect a surface or a gas. The consequences of the foregoing are that in order to disinfect a surface or a gas with ozone: (a) the surface or the gas to be disinfected must be in an enclosed space, which means the disinfection process must be a batch process, not a continuous process, (b) the disinfection process will require significant time, (c) unprotected humans may not be present in the enclosed space during the disinfection process, and (d) the gases venting from the enclosed space during the disinfection process and at the end of the process must be passed through a catalytic converter or some other means that removes any residual ozone in the exhaust gas.
According to an article titled “Demonstration of a Hermetic Airborne Ozone Disinfection System Studies on E. coli” published in the March/April 2003 issue of the American Industrial Hygiene Association, a six-log (base 10) reduction in a microbial population (the common definition of sterilization) can be achieved by exposing a microbial population to 1 to 3 parts per million of ozone for four hours. The U.S. Occupational Safety and Health Administration (OSHA) has set the Public Health Air Standard limit for exposure to 0.1 parts per million of ozone at eight hours and to 15 minutes for exposure to 0.3 parts per million of ozone. Because of the foregoing limitations, it has not been practical to disinfect a surface or a gas with ozone.
Ozone is slightly soluble in water, and, when dissolved in water, will quickly decompose to form the free radicals hydroxide (OH.) and hydrogen dioxide (HO2.) which in turn form hydrogen peroxide (H2O2). Hoigne described the hydroxyl free radical chemistry as having three steps: initiation, propagation, and termination. In initiation, free radicals are produced by ozone decomposition. In propagation, oxygen and water are made into free radicals by the initiating radical. In termination, the free radicals are absorbed, and the reaction stops. The propagation chemistry is very complex and hard to measure and is modified by changes in the water chemistry and the gas chemistry. The initiation chemistry is more straightforward, and scientists agree that the chemistry is:O3+OH−→O2.−+HO2.  1.which indicates the formation of a superoxide radical and a hydroperoxyl radical from ozone;HO2.→O2.−+H+,pKa=4.8  2.which indicates the formation of a superoxide radical. Together, it is obvious that the initiation reactions consume hydroxyl ions and produce protons, a pH-lowering chemistry. If a droplet of ozone and water dropped in pH, it would indirectly indicate that the hydroxyl free radical chemistry had been initiated.
There may be other reactions and reaction end products from putting ozone into water solution, depending on the pH of the water and whether other chemicals are present in the water. Both hydroxide and hydrogen dioxide are strong oxidizers, and each will oxidize any inorganic molecule and any organic molecule that is susceptible to oxidation with which it comes into contact. This is one reason ozone is increasingly being used by public water supply systems as a disinfectant, and also a reason ozonated water is increasing being used as a disinfectant wash for foods, such as fruits, vegetables, and poultry. 21 Code of Federal Regulations 173, Part D, Subsection 173.368 states that ozone may safely be used for the treatment, storage, and processing of foods, including meat and poultry.
The chemistry of ozone-initiated free radicals is very complex. Ozone-initiated free radicals are Nature's way of cleaning up the upper atmosphere (above visible clouds). Ultraviolet (uv) light from the sun converts oxygen into ozone. The ozone is dissolved in <1 μm water droplets along with volatile pollutants. The surface tension of a droplet increases the pressure inside the droplet (P=(272 dyn/cm)/droplet radius). A 1-μm diameter droplet has an internal pressure at sea level of ˜70 psia; a 0.5-μm diameter droplet has an internal pressure of 140 psia. At these very high pressures, the light-induced ozone is converted into a variety of free radicals that clean the atmosphere.
The term “ozonated water” is used herein to refer to water containing the end products of the chemical reaction of ozone with water. The composition of the end products will vary depending on several factors, principally the pH of the water and whether the water contains other dissolved chemical compounds.
If ozone were highly soluble in water, one could easily disinfect a surface by simply dissolving ozone into water to a sufficiently high concentration, allowing the ozone to react with the water to form hydroxide, hydrogen dioxide, and other free radical reaction products and then spraying or swabbing the ozonated water onto the surface and one could disinfect a gas by simply spraying the ozonated water into the gas as a fine mist. The ozonated water, if allowed to wet the biological contaminants on the surface or floating in the gas for a sufficiently long time, will first oxidize the outer surface of the contaminants, thereby killing the biological contaminants, and, if the biological contaminates are exposed to the ozonated water for a sufficiently long time, the ozonated water will eventually oxidize the biological contaminants, thereby eliminating them.
Because ozone is not readily soluble in water, however, simply bubbling ozone through a water column will not result in sufficient ozone being absorbed into the water to produce hydroxide and hydrogen dioxide and other free radical reaction end products at a sufficient concentration for disinfection purposes other than the disinfection of the water itself.
One of the present inventors has described a three-fluid nozzle that uses compressed air as a motive force, a low pressure side stream of ozone gas, and a thin film of water to make fine droplets (Resch et al., U.S. Pat. Nos. 6,076,748 and 5,337,962, both of which are incorporated hereinto by reference). The '748 patent teaches that the water is stretched into ribbons, the ribbons increasing the solubility of ozone in the water, with the ribbons then fragmenting into droplets. There is considerable droplet size distribution, with the number median diameter about 3 μm. The mass median diameter is an order of magnitude larger, indicating the presence of very large particles in the distribution.
Another of the present inventors has disclosed a method of making very small bubbles in water (Bettle, U.S. Pat. No. 5,772,886). The '886 patent teaches that impinging a contained gas/liquid stream against another contained gas/liquid stream at combined velocities greater than 7 ft/sec fractionates the bubbles to less than 1 μm. Extremely small bubbles do not float out of the surrounding water.
It has been reported by Cho (“Disinfection of Water Containing Natural Organic Matter by Using Ozone-initiated Radical Reactions” (Appl. Environ. Microbiol., 2003 April; 2284-2291) that the CT (concentration*time) “value of hydroxyl radicals for a two-log reduction of B. subtilis was estimated to be about 2.4×104 times smaller than that of ozone and was 106 and 105 times lower than those of free chlorine and chlorine dioxide, respectively.” Thus Cho teaches that hydroxyl radicals are many orders of magnitude faster disinfectants than ozone, chlorine, and chlorine dioxide.
Owing to a well-known need to disinfect a plurality of areas of the environment, it would therefore be advantageous to provide a high-concentration composition comprising free radicals for disinfection purposes, and a method of making same. It would also be advantageous to provide such a composition in a mist form, and preferably in a form that does not substantially wet contacted surfaces.